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Glycolysis is the first stage of aerobic glucose metabolism, where a single molecule of glucose ( C_6H_{12}O_6 ) is broken down into two molecules of pyruvate ( C_3H_4O_3 ). This process occurs in the cytoplasm of the cell and does not require oxygen, making it an anaerobic process. However, it is crucial for aerobic metabolism because it provides the initial substrates for further pathways.
During glycolysis, energy is extracted in the form of ATP and NADH. Specifically, two molecules of ATP are used up in the early steps to energize glucose, and four ATP molecules are produced later, which results in a net gain of two ATP molecules per glucose molecule. Additionally, two molecules of NAD^+ are reduced to NADH, capturing high-energy electrons for use in further stages of metabolism.
Key Points:

• Splits one glucose into two pyruvate
• Net gain of 2 ATP and 2 NADH
• Occurs in the cytoplasm and is anaerobic

The Citric Acid Cycle, also known as the Krebs cycle or TCA cycle, is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. This cycle takes place in the mitochondria of cells.
In this cycle, each acetyl-CoA combines with a four-carbon molecule, oxaloacetate, to form a six-carbon citrate molecule. Through a series of reactions, this citrate molecule is broken down again to oxaloacetate, releasing two molecules of CO_2 , and transferring energy to the carrier molecules NAD^+ and FAD, forming NADH and FADH_2.
Summary of Outputs:

• Produces 2 CO_2 molecules per acetyl-CoA
• Generates 3 NADH, 1 FADH_2, and 1 GTP or ATP per cycle
• Cycle completes twice per glucose molecule because one glucose creates two acetyl-CoA

Before pyruvate can enter the Citric Acid Cycle, it must first be converted into acetyl-CoA, a critical junction point for cellular respiration. This conversion occurs in the mitochondrion, involving decarboxylation, which removes one carbon from pyruvate as CO_2 .
The pyruvate is then oxidized, transferring electrons to NAD^+ to form NADH. The remaining two-carbon fragment is linked to coenzyme A, forming acetyl-CoA. This step is crucial, linking glycolysis to the Citric Acid Cycle and thus connecting carbohydrate metabolism to the electron transport chain.
Key Reactions:

• One CO_2 is released per pyruvate
• Produces 1 NADH per pyruvate
• Forms 1 acetyl-CoA per pyruvate, which feeds into the Citric Acid Cycle
• Occurs in the mitochondria before the Citric Acid Cycle